Multi antenna transmission is expected to become more and more common when increased capacity is needed in telecommunication networks. A number of different multi antenna transmission techniques have been developed such as transmit diversity, beamforming and MIMO (Multiple Input Multiple Output).
Transmit diversity is a multi antenna solution in which an information-bearing signal is transmitted from several antennas along different propagation paths. Transmit diversity may for instance help to overcome the effect of fading, i.e. distortion of the information bearing signals. When using transmit diversity the amount of received signal improvement among others depends on the independence of the transmission characteristics for the signals sent from the different transmission antennas. In the current release of the WCDMA (Wideband Code Division Multiple Access) specification (Rel-6) two different modes of transmit diversity are specified, open loop Space-Time multiple antenna Transmit Diversity (STTD) and a Closed-Loop mode multiple antenna Transmit Diversity (CLTD).
Beamforming using multiple antennas for transmission is a signal processing technique which uses arrays of transmitting antennas that control the directionality of a radiation pattern. When transmitting a signal, beamforming can increase the power in the direction the signal is to be sent and at the same time put a null or minimize the power towards unwanted directions. The change compared with an omnidirectional transmission is known as the array gain. These changes are done by creating beams and nulls in the radiation pattern.
MIMO, refers to the use of multiple antennas both at the transmitter and receiver. MIMO performs spatial information processing with multiple antennas. MIMO technology has attracted attention in wireless communications, since it may offer significant increases in signals throughput and link range without additional bandwidth or transmit power. It achieves this by multiple data stream transmission creating higher spectral efficiency (i.e. more bits per second per Hertz of bandwidth) and link reliability or diversity (reduced fading).
When multiple transmit antennas are deployed at a base station several power amplified parallel signals are needed. One common deployment is to equip each transmit branch with its own power amplifier (PA). In a base station with two transmit antennas each provided its own PA, signals will be sent from both antennas. When multi antenna transmit techniques such as transmit diversity or MIMO is applied, both PAs are loaded. However, these multi antenna techniques may not be applicable to all channels. For example, MIMO is only applicable to the HS-DSCH (High Speed Downlink Shared Channel) in a Wideband Code Division Multiple Access (WCDMA) system, while dedicated channels may be transmitted using only one transmit antenna. Similar problems may exist with transmit diversity. Some channels do not gain by using multiple antenna transmission, this is especially true if the channel is dispersive. It will be shown that multiple antenna transmission might reduce the throughput of HSDPA (High Speed Data Packet Access), and the natural choice might therefore in some situations be to not transmit to HSDPA-users with multiple antenna transmission.
FIG. 1 is a schematic block diagram of a transmit unit 1 according to prior art. In this example the transmit unit 1 is a Node B in a WCDMA system. The transmit unit has a first antenna 2 and a second antenna 3. The first antenna 2 and the second antenna 3 have their own power amplifiers, so in this example there are two power amplifiers, 7 and 8, one per antenna. The transmit unit 1 transmits signals from both antennas. From the first antenna 2 the transmit unit transmits signals using single antenna transmission 9. These single antenna transmission signals 9 are schematically illustrated in FIG. 1 by dashed and dotted lines. From the first antenna 2 and second antenna 3 the transmit unit 1 transmits signals using multiple antenna transmission 10, which are schematically illustrated in FIG. 1 by dashed lines. In this example the signals that are transmitted from both the first antenna 2 and the second antenna 3 are transmitted using transmit diversity as multiple antenna transmission technique but the signals could also be transmitted by using e.g. MIMO.
In FIG. 1 three mobile terminals 4, 5 and 6 are illustrated. The mobile terminals 5 and 6 are configured to receive the signals 9 transmitted with single antenna transmission from the first antenna 2. The mobile terminal 4 is configured to receive the signals 10 transmitted with multiple antenna transmission from both the first antenna 2 and the second antenna 3.
The mobile terminals 4, 5 and 6 need a pilot channel as a reference for channel estimation in order to be able to demodulate the received signals. In the WCDMA standard there is a pilot channel specified that is called the primary common pilot channel (P-CPICH), which is the default channel for demodulating a specific channel in a WCDMA system. The P-CPICH is a downlink physical channel. In case multiple antenna transmission is used on any downlink channel in a cell, the P-CPICH shall be transmitted from both antennas using the same channelization and scrambling code. However in the case with multiple antenna transmission, there is a predefined bit sequence added on the P-CPICH that is transmitted from the second antenna 3.
FIG. 2 shows the power distribution for the two power amplifiers 7 and 8 in the transmit unit 1 in FIG. 1. The transmission of signals using multi antenna transmission 10 from both the first antenna 2 and the second antenna 3 and the transmission of single antenna transmission 9 from the first antenna 2 will result in an unequal power load on the power amplifier 7 and the power amplifier 8. Channels that are transmitted with multi antenna transmission 10 by using the P-CPICH 11 and P-CPICH 12 that are transmitted from both the first antenna 2 and the second antenna 3 will result in a power load 20 on the power amplifier 7 and a power load 22 on the power amplifier 8. Channels that are transmitted with single antenna transmission 9 from the first antenna 2 will result in a power load 21 on the power amplifier 7. The total power load 23 on the power amplifier 7 is therefore higher than the power load 22 on the power amplifier 8. This unequal power load on the power amplifier 7 and on the power amplifier 8 will result in a very inefficient use of the power amplifier resources 7 and 8 in the transmit unit 1 in FIG. 1. The power amplifier 8 will only be utilized to a fraction of its potential.
One solution to the above mentioned problem is to equip the PA resources with a load balancing network. This can be done by e.g. introducing Butler matrices before and after the PA. It is also possible to weight the signals digitally at e.g. the baseband so that they are loading all PAs, and the inverse operation can then be done at the radio frequency level after the PA. The problem with this type of load balancing is that it will likely introduce additional power loss from the load balancing network and it requires as well a replacement of equipment at the cell site, e.g. at technology migration. This will of course increase the cost of deploying e.g. MIMO and the cost of the system.
The European patent application EP 1617570 describes a transmit diversity scheme where the transmission modes can be switched between transmit diversity mode and non diversity mode. The total base station power is generally equally split between the transmit antennas regardless of whether the transmission is in transmit diversity mode or in no transmit diversity mode. Moreover, an additional transmit diversity pilot is only sent when the transmission is in diversity mode. That is, when transmitting in the no transmit diversity mode, no additional diversity pilot is sent, resulting in resource savings.
One of the drawbacks with the prior art solution described in the above mentioned European patent application for load balancing between the two transmit antennas is that it does not provide a mechanism for load balancing when mixing multiple antenna transmission with single antenna transmission. The described prior art mechanism is only concerned with load balancing when the transmission modes are switched between transmit diversity mode and multiple antenna non diversity mode.
Further, in HSDPA the HS-DSCH is shared between mobile terminals using channel-dependent scheduling to take advantage of favorable channel conditions to make best use of available radio conditions. This feature is called multi-user scheduling and obtains a diversity gain since the users with most favorable radio channels can be scheduled in each TTI (Transmission Time Interval). Each mobile terminal periodically transmits an indication of the downlink signal quality. The Node B uses this information received from all mobile terminals to decide which mobile terminals will be sent data on the next 2 ms sub-frame. The allocation of radio resources is done in units of channelization codes, of which 16 channelization codes exist, and of which up to 15 can be allocated for HSDPA-transmission. If a mobile terminal is allocated many channelization codes in a TTI more data can be transmitted to that mobile terminal than if the mobile terminal was allocated fewer channelization codes. The HS-DSCH is not power controlled to obtain a certain transmission quality. Instead the coding and modulation are adapted to follow the fast fading and to obtain a certain transmission quality. The Node B will select the correct modulation and coding scheme for each TTI. The Node B will select a higher modulation level (e.g. 16 QAM Quadrature Amplitude Modulation) for mobile terminals with a better radio channel and a lower modulation level (e.g. QPSK Quadrature Phase-Shift Keying) for the other mobile terminals. 16 QAM uses four bits to represent a symbol, while QPSK uses two bits for the same purpose. Hence 16 QAM doubles the data rate compared to QPSK. The power on the HSDPA channel may however change since one power allocation scheme in HSDPA is to use the remaining power when power allocation to dedicated channels have been done. This means that the HSDPA power would be changing since the power of the dedicated channels are changing.
As mentioned above, it is well known that transmit diversity may harm the throughput in HSDPA. This depends on that transmit diversity will lower the peaks of the fading and hence the data rate of the scheduled user. The dynamic range of the received signal is larger without transmit diversity than with transmit diversity and users can therefore be scheduled at a higher signal level. If the peaks are lower a lower modulation level must be used and hence the data rate is lowered. This effect is illustrated in FIG. 6 where solid lines 60 indicate the fading pattern without transmit diversity while the dash-dotted lines 61 indicate the fading pattern when transmit diversity is applied. It is further illustrated that a considerable gain can be achieved if only fading peaks are used for scheduling data to a particular user and thus avoiding inefficient transmission at fading dips. Therefore, it would also be desirable to be able to utilize existing multiple antenna units to balance the load when only single antenna transmission is used.